1. Field of the Invention
This present invention generally relates to Aluminum-Copper-Lithium-Magnesium based alloy products.
2. Description of Related Art
In order to aggressively reduce aircraft weight for better fuel efficiency, low density aluminum-lithium alloys are being assertively pursued by airframe manufacturers and aluminum material manufacturers.
When it comes to sheet products used in aircraft applications, aircraft designers generally use either “medium strength—high damage tolerance” alloys like AA2024 alloy and its recent derivatives like 2524 (see for example U.S. Pat. No. 5,213,639), or “high strength—medium damage tolerance” alloys like AA7075 alloy.
For both types of alloys (i.e. AA2024 type alloys or AA7075 type alloys), there are additional requirements to be fulfilled in order to be used by the aircraft industry. For instance, better formability is required in order to produce the complex parts needed on an aircraft and a better corrosion resistance than incumbent alloys is desired for lower aircraft maintenance and operation cost.
If there has been a considerable amount of works related to low density, Al—Li based alloys alternatives to AA2024 type alloys (i.e. medium strength—high damage tolerance), limited Al—Li based product has been developed to provide aircraft designers with better alternatives than currently used high strength 7075 sheet.
The strength of Al—Li sheet is critical for aerospace applications. The higher strength allows less total weight component design for better fuel efficiency. As a reference, the yield strength of commonly used 7075-T6 aluminum alloy at about 0.05″ thickness sheet is 68 ksi based on “Aluminum Standards and Data 2013” published by The Aluminum Association. Most of the current Al—Li sheet alloys have very low strength compared with 7xxx sheet.
It is also well known that it is an extreme metallurgical and technical challenge to produce aluminum-lithium (Al—Li) product, especially very thin sheet products, in which the material strength, formability, fracture toughness, fatigue resistance, and corrosion resistance are required simultaneously.
Metallurgically, the desired microstructure and texture, which strongly affect the final product properties, are much more difficult to control for sheet, especially thin sheet, Al—Li products. The microstructure and texture are strongly affected by chemical composition of the alloy and most of the manufacturing steps, i.e. homogenization, hot and cold rolling, annealing, solution heat treatment, and stretching. Al—Li sheet, especially thin sheet, is much more difficult to manufacture than conventional alloy: thin Al—Li sheets are more sensitive to rolling cracking, surface oxidation, and distortion. Due to these limitations, there is a small processing window that can be used to optimize the desired microstructure and texture. Therefore, this is a significant challenge to design an aluminum-lithium sheet alloy which achieves the desired combination of properties (strength, formability, cost, with good damage tolerance and corrosion resistance). These fabrication technical challenges restrict a lot the production of high strength thin sheet Al—Li product.
As a consequence, there is only one Al—Li alloy, i.e. AA2090, registered for sheet products with a thickness less than 0.063″, and only one additional alloy, i.e. AA2198, registered for sheet products with a thickness less than 0.125″, and only two additional alloys, i.e. AA2195 and AA2199, registered for sheet/plate products with a thickness less than 0.5″, based on the most recently (2011) published “Registration Record Series—Tempers for Aluminum and Aluminum Alloys Production” by The Aluminum Association.
These metallurgical and technical challenges for producing high strength thin sheet products are also reflected in the patents and patent applications. In fact, a significant amount of patents or patent applications are mostly related to plate products (>0.5″), but only a few to sheet products.
The cost of Al—Li alloy product is another concern. Silver (Ag) element is added to many new generation Al—Li alloys in order to improve the final product properties, adding significant alloy costs. Among those four registered Al—Li alloys sheet products mentioned previously, two of them (AA2198 and AA2195) are Ag containing alloys.
U.S. Pat. No. 7,744,704 discloses an aluminum-lithium alloy for aircraft fuselage sheet or light-gauge plate applications. This patent is the basis for the registered AA2198 Al—Li sheet alloy. This alloy comprises 0.1 to 0.8 wt. % Ag, so it is not considered to be a low cost alloy. Furthermore it has a relatively low strength compared to 7075 T6 sheets.
U.S. Pat. No. 7,438,772 discloses an aluminum-copper-magnesium alloy having ancillary additions of lithium. This patent is the basis for registered AA2060 Al—Li alloy. The claimed level for lithium is only from 0.01 to 0.8 wt. %; because of this limited addition of lithium, this is not considered to be really a “low-density” alloy.
U.S. Pat. No. 8,118,950 discloses improved aluminum-copper-lithium alloys. This patent is the basis for registered AA2055 Al—Li alloy. This alloy comprises 0.3 to 0.7 wt. % Ag, so it is not considered to be a low cost alloy. As provided in the patent, the alloy is used for high-strength extrusions.
U.S. Pat. No. 7,229,509 discloses an alloy with a broad chemical composition range, and including 0.2 to 0.8 wt. % Ag, so it is not considered to be a low-cost alloy. This patent is the basis for registered AA2050 Al—Li plate alloy. As described in the paper of “Aluminum-Copper-Lithium Alloy 2050 Developed for Medium to Thick Plate [Lequeu 2010]”, AA2050 is designed for Al—Li plate products from 12.7 mm (0.5″) to 127 mm (5′). Similar to patent U.S. Pat. No. 7,229,509, patent application of “US20110209801 A2” includes 0.15 to 0.35 wt. % Ag. In addition, this application specifically claims that the alloy is suitable for plate in thickness range of 30 mm (1.2″) to 100 mm (3.9″).
Other patent applications that includes Ag and are also used for thick plates are “US 2009/0142222 A1” and “US 2013/0302206”.
U.S. Pat. No. 5,032,359 discloses an alloy including 0.05 to 1.2 wt. % Ag, so it is not considered to be a low-cost alloy. The main advantage of this alloy is to have high strength, ductility, excellent weldability, and natural aging response.
Patent application of “US 2014/0050936 A1” discloses an Al—Li alloy product containing 3.00 to 3.80 wt. % Cu, 0.05 to 0.35 wt. % Mg, and 0.975 to 1.385 wt. % Li. This is basically an Al—Li version of “high damage tolerance—medium strength” application alloy, with strength not matching the AA7075 performance.
In general, the current related prior art teaches that (1) there is a strong need for high strength, low density, high formability, low cost, together with good damage tolerance and corrosion properties, Al—Li alloys capable of producing thin sheet products; (2) it is an extreme metallurgical and technical challenge to produce such products; (3) the very expensive Ag is often added for better metallurgical quality, but this addition significantly increases the Al—Li product cost.